EXPLANATION OF THE PLATES. 
331 
PLATE IV. 
Fig. 1. is a section of the parts of a high pressure engine with a four-passaged 
cock. The engine is supposed to be partly within the boiler, of which D B is the 
top plate. P is the steam piston, and R the piston rod, A is the four-passaged 
cock; the steam enters from the boiler at S, and passes through t to the top of the 
piston, and the steam below escapes through the passage b, and pipe a and E, to 
the atmosphere; the pipe E is surrounded by water, which the escaping steam 
warms ready for the boiler. By turning the cock the motions are reversed, but it 
is obvious we cannot in this engine employ the expanding force of the steam. The 
motion is regulated by a throttle valve V. See art. 356—361. 
Fig. 2. and 3. show a section and plan of a similar engine, with a D-slide 
instead of a cock ; the steam enters from the boiler at S, and by the passages being 
shut and opened close to the extremities of the cylinder, there is no loss by the 
communicating pipes being filled with strong steam. See art. 364. This engine 
will not work expansively unless the construction of the slide be altered. See 
art. 371. Contrary to the usual practice, the packing of the slide is on the sliding 
part; the advantage of this plan is obvious, but the practical difficulty of boring 
a semi-cylinder is incurred. 
Fig. 4. is a simple arrangement of the high pressure engine by which the 
expanding power of the steam may be used ; it is the invention of Murdoch. The 
passages are opened and closed by pistons sliding in a pipe : the steam enters this 
pipe at S, and the steam is supposed to be just shut off by the upper piston, so that 
by the expansion of that in the cylinder the rest of the stroke is completed, the 
passage E to the atmosphere being still open. See art. 371—380. The slide 
would be improved by making it of the form of a D-slide. 
The construction of the pistons of the slide is a suggestion which may perhaps 
answer better than the common ones, (art. 450. and note.) 
Fig. 5. is an arrangement to illustrate the action of a high pressure engine to 
work expansively by means of combined cylinders. See art. 381—383.